The present invention relates to paving materials and methods, and in particular to modified asphalt compositions and methods for making and using them.
Asphalt is a viscous material that is derived from crude petroleum and is used in paving roads. Asphalt is generally understood to include asphaltenes, resins, and oils. While asphalt is primarily composed of hydrocarbon molecules (hydrogen and carbon), it also contains elements such as oxygen, nitrogen, and sulfur. Asphalt has an extremely diverse molecular structure depending on the crude source. Asphalts are thermoplastic materials--that is, they gradually liquefy when heated. Asphalts are characterized by their stiffness, consistency, or ability to flow at different temperatures.
Asphalt that has been specially prepared for use in pavement by controlling its quality and consistency is called asphalt cement. Asphalt cement is ordinarily used in a "hot mix" pavement composition that also contains coarse and fine aggregate. This composition, also called asphalt concrete, is blended at high temperature, applied to the roadbed, and compacted with rollers to produce a smooth driving surface.
Asphalt cement maybe modified by the addition of components that increase the strength of the material or otherwise alter its properties. In particular, it has become known in the art to add carbon black to asphalt as a filler material. It has also become known that one may add polymers to an asphalt blend in order to improve its strength and reduce its temperature susceptibility. Modification of asphalt with a variety of different polymers, including natural and synthetic rubber, acrylates, and styrene-acrylic copolymers has been described. Because the polymer modifiers that have been employed most beneficially as asphalt modifiers are rather expensive, a need exists for alternative, lower-cost modifiers that nonetheless impart improved properties comparable to those achieved by using the more expensive polymers.
Another pressing concern is a growing problem with solid waste disposal in general. The volume of solid waste generated annually in this country is growing steadily, while at the same time the available capacity of disposal sites is shrinking. This concern has led to a variety of initiatives for recycling. In 1991, for example, Congress mandated that state highway agencies investigate possible methods for using recycled materials in road construction. A number of different waste materials, including glass, shingles, shredded brush, shredded rubber tires, and a variety of other plastics have reportedly been blended into asphalt pavements in response to this mandate.
The quantity of waste toner currently being generated for landfill disposal raises particular concerns. Toner is the dry ink used in laser printers, copiers, and fax machines. Toner is a fine black powder mainly composed of styrene-acrylate copolymer and iron oxide. Some toners consist of more than 80 to 90 percent styrene-acrylate copolymer, about 5 to 10 percent carbon black, and a small amount of iron oxide. Some others, called single component toners, consist of about 40 to 50 percent styrene-acrylate copolymer, 40 to 50 percent iron oxide, and a small amount of carbon black and other ingredients. Approximately less than 5 percent of the toner may consist of polypropylene, silica, organic pigment, ferrite, and some additives.
The melting point of toner varies depending on the type and ranges between 100 to 150.degree. C. In general, the toner specific gravity ranges between 1 and 1.7 depending on the type.
Waste toner is derived from two principal sources: material not sold by the toner manufacturer because it fails to meet manufacturing specifications, and post-consumer residual material from spent toner cartridges, printers, and copiers. Most used toner cannot be directly recycled and must be discarded. Although it is possible to incinerate waste toner to recover significant heat energy, most waste toner is disposed of by burial in landfills. Like most materials that are disposed of in landfills, waste toner does not degrade. Although estimates vary, the amount of waste toner produced and landfilled annually is roughly 9,000 to 25,000 tons. At a landfill disposal cost of at least $70/ton, this quantity of discarded waste toner costs the toner industry between $600,000 and $2,000,000 per year. This makes it especially desirable to develop alternative, more cost-effective methods for disposing of this material, and particularly methods for recycling it.
The use of asphalt in road building has gradually increased through the years and reached its peak in 1979. It is speculated that the current annual worldwide consumption of asphalt is over 100 million tons. Currently in the United States, the annual consumption is at least 25 million tons. Approximately 94 percent of U.S. roads are paved with asphalt.
Asphalt is used in road paving construction because it is a strong cement, readily adhesive, highly waterproof, and durable. It is a plastic substance that imparts controllable flexibility to mixtures of mineral aggregates with which it is usually combined. It is because of these superior qualities that asphalt is so widely used in road construction.